Depends on what is under the backerboard as it offers little in structural strength, its real purpose is too offer a superior surface for the tile to be bonded to. The subflooring must be adequate as well as the size and spacing of the floor joists as well as the length of the unsupported span. If these are not up to snuff neither a mud job, which needs to be at least 1 & 1/4" thick throughout the floor, nor the cement board will offer any protection against tile failure. You need to check with a structural engineer that your floor meets a L/720 requirement for stone tiles. They will know what that means.

The joist are 2X8 (16" oc) and the existing subfloor is 3 1/2" tounge and groove planks (perpendicular to the joist) that I believe are 3/4" thick. The open span beneth the subfloor and between the joist is 13' - 15' (the room is not square by design).

I have an existing ceramic tile floor with a mudd base over the same subfloor for over 8 years and have had no tile failure. However, I will take your advise and consult a structural engineer.

If the additional structural info makes a difference any and all insight/info would be greatly appreciated.